Decomposition of Diazenes and Diazene N-Oxides. Consideration of Three-Electron Stabilization.

Abstract

The carbon-nitrogen bond is a strong link of average bond energy 75 kcal/mole. There are, of course, many examples in which homolytic cleavage of this bond may be effected under relatively mild conditions. Diazenes (azo compounds) comprise a well-documented example (1). Decomposition of diazenes affords dinitrogen and radicals. The question of whether the rate-determining step involves synchronous two-bond cleavage (eq. 1) or one-bond cleavage (eq. 2) has received a great deal of study (1, 2). For alkyldlazenes (Including aryl-substituted alkyl) the evidence strongly favors cleavage of both carbon-nitrogen bonds in the rate-determining step. A convincing line of evidence is the dependence of rate on substituent [e.g. Table 1 (3)]. Table 1 Rate of Decomposition of Diazenes, Ra-N=N-Rb, rel k, predicted,200°C Indeed, these findings might appear to be a logical consequence of a consideration of the energetics of the reaction. The enthalpy change, ΔH°, for eq. 1 is approximately +25 kcal/mole for R = alkyl, e.g. t-butyl (4); the enthalpy of activation